1. Field of the Invention
The invention relates to a device for holding a beam splitter element having an optically active beam splitter layer in an optical imaging device, the beam splitter element being connected to at least one support element that is fastened in the housing of the imaging device.
2. Description of the Related Art
In projection exposure machines for microlithography having catadioptric projection objectives that have a polarizing beam splitter cube, the beam splitter cube deflects the light beam coming from the reticle into a cantilever arm with a concave mirror, while the light beam returning from the cantilever arm is let directly through the beam splitter cube. In this case, temperature changes during the ongoing operation result, in particular, from the absorption of light in the beam splitter layer of the beam splitter cube, but also from that in other layers as well as in the entire volume of the beam splitter cube. It is known that a rise in temperature results in material expansion and associated changes in the geometry of the beam splitter cube depending on the mounting of the latter. In addition, because of the fact that radiation passes through them repeatedly, specific regions of the beam splitter cube are heated more strongly than other regions.
Known designs use the beam splitter cube with the beam splitter layer vertically, i.e. in the direction of the gravity. Furthermore the volume of the beam splitter cube in such systems is much higher and the power of the laser used is much lower. Since the bearing is applied to a side face of the cube, the place for the bearing can be chosen freely and there is no area, through which radiation passes, that may not be used for the bearing. Most known systems use a beam splitter cube made of quartz comprising much lower thermal expansion than calcium fluoride (CaF2), which is presently used. In the case of mounts, which hold such a beam splitter cube in the region of one or more of its side faces, undesired movements such as displacements or rotations of the beam splitter layer in the interior of the beam splitter cube, can occur. This movement, which stems, from the length expansions of the material of the beam splitter cube and, on the other hand, from deformations of the beam splitter layer owing to thermally induced stresses, worsens the imaging behavior of the catadioptric projection objective of the projection exposure machine. The thermal expansion of the beam splitter cube material can tilt and displace the plane of the beam splitter layer such that the light beam coming from the reticle is no longer reflected exactly into the cantilever arm. In addition, the beam splitter cube and the beam splitter layer can also themselves be deformed such that not only is the beam wrongly deflected, but aberrations occur in the imaging of the projection objective.
Such undesired changes of the beam splitter layer can also occur, of course, in the event of global warming in the objective.